A steady, but tested, friendship dating back to the 1990’s could be affected again if Biden joins the Democratic primary.

Joe Biden had kind words for Hillary Clinton in 2008, when he was the Democratic vice presidential nominee and she was campaigning for the Biden-Obama ticket around the country.

“This is a woman who understands what’s right and what’s wrong, what’s fair and is unfair, and is always, always fighting against the abuse of power by the powerful,” Biden said at a rally in Scranton, Pa.

Biden even said at one point that Clinton might have been a better choice as Barack Obama’s running mate.

The two have a long history together, as political rivals as well as friends. Now Biden must decide whether to test that friendship by challenging Clinton for the 2016 Democratic presidential nomination.

A July 9-12 Monmouth University Poll suggests support for Biden would come largely at Clinton’s expense, because her supporters are ideologically similar to his.

Thirteen percent of Democratic voters said they would support Biden over Clinton and the other four Democratic candidates already in the race. When asked only about Biden, an additional 43 percent of respondents said they would be very likely or somewhat likely to support him over their current choice if he ran. Of that group, 68 percent gave Clinton as their current choice.

“He’d walk into the polls with at least a quarter of support, with the potential for going up from there,” Patrick Murray, director of the Monmouth University Polling Institute, said. “That would probably knock Clinton down to below 40 percent.”

Biden has said Clinton’s candidacy wouldn’t affect his decision on whether to run, which he said he’d announce by the end of the summer.

Support for Clinton, still the Democratic frontrunner by far, has dropped following reports that she used a private email server for government business while secretary of state. A Quinnipiac University Poll in August showed Biden running as well as or better than Clinton against top Republicans in general-election matchups in Florida, Ohio and Pennsylvania, all key swing states.

As talk of a Biden challenge increased this month, Clinton said in New Hampshire, “I have the highest regard and affection for him. We should all let the vice president be with his family and make whatever decision he believes is right.”

Biden is still grieving over the May 30 death of his son, 46-year-old Beau Biden, Delaware’s former attorney general, from brain cancer.

In deciding on a White House bid, Biden must first consider the needs of his family members, Carper said. Other factors would be his ability to raise money — and the outlook for Clinton, he said.

“I think he and Hillary have been good friends, I think they still are,” Carper said. “I think one of the considerations has to be is whether or not 16 months from now she will be electable.”

Biden’s alliance with Clinton dates to Bill Clinton’s presidency in the late 1990s, when Biden used his influence as the top Democrat on the Senate Judiciary Committee to help pass legislation on crime, gun violence and domestic abuse. Years later, when Hillary Clinton was preparing to enter the Senate, one of her first stops was Biden’s office.

“My sense was they were very respectful of each other and liked each other,” said former Sen. Kent Conrad, D-N.D., who chaired the Senate Budget Committee when Hillary Clinton was a member. “I can remember them sharing conversations, laughing out loud, enjoying each other’s company.”

When Clinton was secretary of state, she and Biden regularly discussed policy issues over breakfast at Biden’s official residence. In June, Clinton flew to Delaware for Beau Biden’s funeral Mass. She has called Joe Biden “a good friend and a great man.”

Their friendship was tested previously when they both ran for the 2008 Democratic presidential nomination. Biden, then chairman of the Senate Foreign Relations Committee, said Clinton’s proposal to cap U.S. troop levels in Iraq and set conditions for aid to Iraqi forces would lead to “disaster.” He also said she was mistaken in saying Pakistani President Pervez Musharraf was up for re-election and that his country hasn’t had a history of peaceful successions.

“How can you lead immediately if you don’t know those facts?” he asked.

Biden and Clinton sat on separate committees and focused on different issues, and Biden’s former Senate aides said Clinton wasn’t a frequent visitor to his office. But Biden’s office bookshelf prominently displayed a photo showing Clinton laughing as Biden whispered in her left ear and Bill Clinton whispered in her right.

“There’s no woman like Hillary Clinton,” Biden said in 2013 as they shared the stage at an awards ceremony.